This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which are capable of detecting and quantifying valvular regurgitant blood flow in the heart.
Valvular regurgitation is a serious and potentially life-threatening heart condition. The condition arises when a valve in the heart does not fully close during a particular phase of the heart cycle. Full valve closure is necessary for a complete build-up of the maximum heart chamber blood pressure developed by contraction of the heart. If a valve of the chamber does not close completely, a leak will occur and a jet of blood will escape as the heart contracts. This inefficient operation will cause the heart to expend more effort than it should, can lead to a reduced flow of blood through the body, and in many cases leads to open heart surgery to repair or replace the leaking valve.
Ultrasonic detection of valvular regurgitation has traditionally been done by looking for the above-mentioned jet of blood. Over the past fifteen years detection of the jet has been facilitated by colorflow Doppler, in which the high speed and turbulence of the small jet of blood can be detected by careful search for these abnormal local flow velocities near the leaking heart valve. More recently a diagnostic procedure known as the proximal iso-velocity surface area method (PISA) has been endorsed by the cardiology community. In this method the suspect valve and the region inside the heart chamber and proximal to the valve are imaged by colorflow Doppler imaging. At the time of occurrence of the jet a flow convergence region (FCR) is formed in the proximal region as blood flow velocities in the region instantaneously accelerate toward the regurgitant orifice. This flow pattern results in aliasing in the colorflow image as the flow velocities momentarily exceed the velocity range used for the colorflow image. A colorflow image at this moment is captured and frozen on the display screen. A measurement is then made of the velocity v at the first aliasing line of the FCR, and a measurement is made of the distance r from the aliasing line to the center of the valve orifice. These two measurement are then used to compute the flow rate through the orifice using the expression Qt=2xcfx80r2v.
Several difficulties arise when conducting this procedure. One is that the greatest accuracy is obtained when the jet is captured in the colorflow image at its very peak. The duration of the jet during a heart cycle can be only 300-450 milliseconds, however, while a typical colorflow frame rate may be in the range of 10-20 frames per second. Thus it is probable that the time of acquisition of one of the colorflow image frames will not be the same as the time that the jet is at its peak. The clinician can repeat the colorflow acquisition sequence for additional cardiac cycles, or can settle for the inaccuracy causes by making the measurements at other than the peak of the jet.
Another problem is that the center of the valve orifice is not easy to define in the colorflow image. The valve tissue produces large reflections of ultrasound and is moving rapidly as scanning takes place, and can appear as a bulky, blurred or indistinct mass in the image. Thus it is possible that the accuracy of the measurement r will be compromised by the inability to estimate the exact location of the orifice.
Yet a third problem is that the PISA method is tedious and exacting, limiting its utility for routine use to measure regurgitant volumes in a clinical setting. The PISA method requires the clinician to make multiple measurements on each image frame of multiple image frames acquired during the regurgitation period of a heart cycle.
It is thus desirable to be able to measure the flow rate and volume at the regurgitant valve without such sources of inaccuracy and inconvenience.
In accordance with the principles of the present invention an ultrasonic system and technique are described for quantifying valvular regurgitant flow rate and volume. In one embodiment of the inventive technique the regurgitant valve is imaged by colorflow imaging. An M-line is positioned over the valve in the image by referencing the position of the jet, the FCR, and/or the approximate location of the orifice. A sequence of color M-mode measurements is then captured along the M-line. The time-sampled FCR during a cardiac cycle is defined in the color M-mode display by its distinctive aliasing color, and the distance across the FCR and velocities at each end of the distance are recorded. The instantaneous flow rate is then calculated from these measurements, and the volume flow during regurgitation is calculated as an integral of the instantaneous flow rates for all the regurgitant M-lines of a cardiac cycle. Since the sample rate of the M-lines can be one or two orders of magnitude greater than the colorflow frame rate, the peak of the jet can be reliably captured, and the inventive technique does not require any precise definition of the orifice location. Moreover, only one image, the color M-mode image, is needed for the diagnosis of a complete heart cycle.